1. Field of Invention
This invention relates generally to the read/write heads used in optical information storage and retrieval systems and, more particularly, to the apparatus used in conjunction with the read/write heads for providing the tracking signals and the focusing signals used in the positioning of the read/write head relative to the storage medium.
2. Description of the Related Art
Referring to FIG. 1, one configuration for an optical information storage and retrieval system according to the related art is shown. A radiation source 11, typically a laser diode, provides a radiation beam which is collimated by collimating lens 12. The collimated radiation beam is transmitted through a polarization beam splitter 13 and applied to a quarter wave plate 14. The polarization beam splitter 13 provides linear polarization for the radiation beam and the quarter wave plate 14 provides a circular polarization to the radiation beam. The circularly polarized radiation beam from the quarter wave plate 14 is focused by objective lens 15 on the information storage surface 10A of the storage medium 10. The storage medium 10 is typically a disk with a surface which interacts with the circularly polarized radiation beam. The interaction with the storage medium surface 10A causes the radiation beam to be reflected and diffracted therefrom. The resulting radiation beam is collimated by objective lens 15 and the collimated resulting radiation beam is transmitted through the quarter wave plate 14. The quarter wave plate restores the linear polarization of the radiation beam. However, a component of the restored polarized radiation beam perpendicular to the polarization of the originally polarized beam will typically be present as a result of the second passage through the quarter wave plate 14. When the restored, polarized radiation beam is applied to the polarization beam splitter 13, the perpendicular component will be reflected by the beam splitter 13. The reflected radiation beam is applied to a sensor focusing lens 16 which converges the resulting radiation beam on sensor array 5. The resulting radiation beam has imposed thereon modulation that can be processed to provide the information (or data) which is stored on the storage medium. In addition, the resulting radiation beam can be processed in such a manner as to provide tracking and focusing signals which can be used to activate apparatus which controls the position of the focused radiation beam on the storage surface 10A (i.e., the tracking in one dimension) and which controls the distance of the objective lens 15 from the storage surface 10A, (i.e., the focusing of the radiation beam on the storage surface). In this type of optical information storage and retrieval system, the quarter wave plate imparts, to the radiation beam illuminating the storage surface 10A, a circular polarization. After reflection from the storage surface, the quarter wave plate restores a linear polarization. However, the linearly polarized radiation beam will have a component which is rotated by an angle of 90.degree. from the plane of polarization originally established by the polarization beam splitter 13. The rotated component of the radiation resulting from interaction with the storage surface 10A is reflected by the beam splitter 13 and applied to sensor array 5.
Referring to FIG. 2, an example of the use of the processing of the radiation beam to provide tracking and focusing signals, according to the related art, is shown. This example is taken from European Patent Application 0,177,108 A1, issued in the name of A. Smid, P. F. Greve, and H. 't Lam, entitled "Opto-Electronic Focussing-Error Detection System, and filed on Feb. 10, 1985. In FIG. 2, the path of the resulting radiation beam, the resulting radiation beam being the radiation beam which has interacted with data track 21, is shown. (The quarter wave plate 14 and the beam splitter 13 have been omitted to emphasize certain important aspects of the configuration.) The data track 10A is the path on the storage surface 10A along which the radiation beam will move in accessing or storing the information encoded on the storage medium 10. A dual prism 25 is shown interposed between the objective lens 15 and the sensor focusing lens 16. The dual prism divides the resulting radiation beam into two components. The two radiation components include the radiation reflected and radiation diffracted from the storage medium. The first component is focused on dual sensor elements A and B of the sensor array 5, while the second radiation beam component is focused on dual sensor elements C and D. As will be known to those skilled in the art of processing resulting radiation beams, the data signal DS, the focusing signal FS, and the tracking signal TS are given respectively by:
DS=A+B+C+D 1.) PA1 FS=(A+D)-(B+C) 2.) PA1 TS=(A+B)-(C+D) 3.)
where A, B, C, and D of the Equations 1-3 represent the voltages developed by the equivalently designated sensor element in response to radiation applied thereto. The data signal DS is the sum of voltages developed by all of the sensor elements. The focusing signal FS is the difference between the sum of the voltages resulting from the radiation applied to a pair of diagonal sensor elements, i.e., A and D, and the sum of the voltages resulting from the complementary diagonal pair of sensors, i.e., B and C. When the absolute value of the focusing signal FS is minimized, the radius of the radiation beam on the storage surface 10A will be minimized, i.e., the radiation beam will be focused on the storage surface 10A. The tracking signal TS is minimized when the radiation reflected and diffracted from above the center of the data track and the radiation reflected and diffracted from the below the center of the data track are equal. In order to understand the origin of the tracking signal, the role of the diffraction of the radiation beam must be understood.
Referring to FIG. 3A, the objective lens 15 is shown focusing the circularly polarized radiation beam on the storage surface 10A of storage medium 10. The storage surface 10A is shown as having a multiplicity of grooves, or equivalently, a multiplicity of data tracks 10B fabricated therein. The grooves 10B have dimensions relative to the wavelength of the radiation beam whereby diffraction patterns are formed. The data tracks 10B can be replaced with series of raised regions which are not connected, can be replaced with regions of appropriate dimension and refractive index, or any other structure which provides diffraction patterns in response to an impinging radiation beam without departing from the scope of the present invention. Referring to FIG. 3B, the resulting radiation beam after interaction with the storage surface is shown. The resulting radiation beam includes a zeroth order (reflected) component and a positive and a negative diffracted component. As will be clear, higher order diffraction components can be present, however, the present invention can be understood without further consideration of these components. The impinging radiation beam is shown as being off center and therefore closer to one edge of the data track or groove which is currently being tracked. This asymmetric positioning causes a wavefront phase shift in the diffracted orders and, consequently, an asymmetric interference between each of the diffracted components and the undiffracted (i.e., reflected or zeroth order radiation component). As a consequence, constructive interference occurs in one region, e.g., the region of overlap between the reflected radiation component and the +diffracted radiation component, while destructive interference occurs between the reflected radiation component and the -1 diffracted component. The magnitude of the resulting signal depends on the amount of shift of the impinging beam relative to the center of the data track or groove. In FIG. 3C, the difference between the intensities of the regions of interference is illustrated by region 32 (wherein the undiffracted radiation component and the +1 first order interference component interfere) and region 34 (wherein the undiffracted radiation beam component and the -1 first order diffracted radiation beam interfere). The polarity depends on whether the tracking of the radiation beam occurs for the data tracks (or grooves) or for the lands, i.e., the regions between the data tracks or grooves. Note that in the preferred embodiment, the two first order diffraction components are contiguous with the optic axis of the radiation beam. As a consequence, the two first order diffraction components will be superimposed on and will interfere with the reflected radiation beam. Referring once again to FIG. 2, the projection of the first order diffraction patterns 29A and 29B are shown on objective lens 15 and on dual prism 25. The difference in intensities of the resulting radiation components separated by dual prism 25 is determined by the relative intensities of the radiation components resulting from the interference between the undiffracted (reflected) radiation component and the first: order diffraction components. It will be clear that the groove can be replaced by diffracted and undiffracted radiation components resulting from applying a radiation beam to a data track without an associated groove, the data track implemented to provide the requisite diffracted and undiffracted radiation components.
The configuration for providing tracking signals and focusing signals, as disclosed by the reference of Smid et al. and described above, suffers from the presence of a significant amount of optical cross-talk, generally originating from ever-present optical wavefront aberrations and the diffracted radiation components. Referring to FIG. 4, an experimental verification of the optical cross-talk between the tracking signal and the focusing signal is illustrated. The presence of this optical cross-talk becomes particularly important in high performance systems, such as are required in the information storage and retrieval systems, i.e., particularly systems requiring high performance servomechanism systems.
A need has therefore been felt for apparatus and an associated method for processing the radiation beam resulting from the interaction with a optical storage medium, having data tracks or grooves, and identifying therefrom the tracking signals and the focusing signals whereby the cross-talk between the two signals is minimized.